NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Convective Cooling and 

Passive Stack 

Improvements in Motors

Kevin Bennion

Principal Investigator

National Renewable Energy Laboratory

Annual Merit Review and Peer Evaluation Meeting

June 18, 2014

Washington, D.C.

PR-5400-61360 Project ID: APE064

This presentation does not contain any proprietary, confidential, or otherwise restricted information.
2

Overview

• Project Start Date: FY 2014

• Project End Date: FY2016

• Percent Complete: 15%

• Cost

• Performance (Power Density) 

• Life

• Total Project Funding:

– DOE Share: $500K

• Funding for FY14: $500K

Timeline

Budget

Barriers

• Motor Industry – R&D Input 

and Application of Research 

Results

– Suppliers, end users, and 

researchers

• Oak Ridge National 

Laboratory (ORNL) – Motor 

R&D Lead

– Tim Burress (ORNL)

– Andy Wereszczak (ORNL)

• National Renewable Energy 

Laboratory (NREL) –

Thermal Project Lead

Partners 

(Interactions/Collaborations)
3

Relevance – Why Motor Cooling?

• Current Density

• Magnet Cost

o Price variability

o Rare-earth materials

• Material Costs

• Reliability

• Efficiency

• Temperature 

Distribution

Stator 

Laminations

Slot-Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End-Winding

Motor Axis

Motor Cooling Section View

ATF: Automatic Transmission Fluid
4

Relevance – Research Objective

Problem

Core Thermal Capabilities 

and Research Tasks

Objective

Support broad industry demand 

for data, analysis methods, and 

experimental techniques to 

improve and better understand 

motor thermal management

Stator 

Laminations

Slot-Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End-Winding

Motor Axis

Motor Cooling Section View
5

Stator 

Laminations

Slot-Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End-Winding

Motor Axis

Approach/Strategy – Problem

Problem

• Extracting heat from within the 

motor to protect motor and enable 

high power density

Examples

1. Orthotropic (direction dependent) 

thermal conductivity of lamination 

stacks

2. Orthotropic thermal conductivity of 

slot-windings

3. Orthotropic thermal conductivity of 

end-windings

4. Convective heat transfer coefficients 

for ATF cooling 

5. Thermal contact resistance of statorcase contact

6. Cooling jacket performance

4

1

1

2 3

5

6

Motor Cooling Section View
6

Approach/Strategy – Focus 

• Measure convective heat transfer 

coefficients for ATF cooling of endwindings

• Develop computational fluid 

dynamics (CFD) models for ATF jet 

impingement

• Measure interface thermal 

resistances and orthotropic thermal 

conductivity of materials

Support broad industry demand for data 

to improve and better understand motor 

thermal management

Apply core thermal experimental and 

modeling capabilities

Objective

Core 

Capabilities

Tasks

Automatic Transmission Fluid Heat Transfer

Computational Fluid Dynamics and Modeling

Material and Thermal Interface Testing

Photo Credit: Jana Jeffers, NREL

Photo Credit: Justin Cousineau, NREL
7

Develop heat transfer correlations and CFD models for 

ATF impingement (building on previous work in FY13)

Approach/Strategy – Plan

2013

Oct Nov Dec

2014

Jan Feb Mar Apr May Jun Jul Aug Sep

Measure local heat transfer coefficients using ATF impingement

Build experimental setup for heat 

transfer experiments on end-winding 

surfaces

Milestone 

Annual 

Report

Thermal measurements of passive thermal design elements and 

collaboration supporting motor research performed at ORNL

• Includes collaboration between ORNL, UQM, and NREL on materials for 

improved end winding heat transfer

Go/ 

No-Go

Measure heat 

transfer coefficients 

on end-windings

Go/ 

No-Go Complete lamination thermal 

tests for publication

Go/ 

No-Go

Milestone

Measure orthotropic 

thermal conductivity 

of ORNL laminations 

and winding samples

• Verify test procedure

• Confirm modeling approach

Go/ 

No-Go Obtain ATF fluid 

properties

Complete In Progress
8

Milestones

Date Description

December 2013 Go/No-Go

• Measure orthotropic thermal conductivity of ORNL laminations and winding 

samples

• Verified experimental and modeling methods

January 2014 Milestone (internal)

• Completed lamination thermal tests for publication

• Began preparation of publication

February 2014 Go/No-Go

• Received ATF fluid property data from Ford Motor Company

• Supports future task to develop heat transfer correlations and CFD models

July 2014 Go/No-Go

• Measure local heat transfer coefficients using ATF impingement

• Supports future task to develop heat transfer correlations and CFD models

August 2014 Go/No-Go

• Build experimental setup for heat transfer experiments on end-winding 

surfaces

• Supports future task to measure heat transfer coefficients on end-windings

September 2014 Milestone report

• Project summary report for FY14
9

Technical Accomplishments

D (mm) d (mm) S (mm) S /d D/d

12.7 2.06 10 5 6.2

Test Sample

Nozzle Plate: 

Orifice Diameter 

(d)=2 mm

Sample Holder/Insulation

Resistance 

Heater Assembly

Fluid Inlet

Aluminum 

Vessel

Thermocouples

S=10 mm

D=12.7 mm

Oil Impingement Test Section Schematic (left). Photo During Operation (right).

• ATF Impingement Test Section

FY13 Related Accomplishments 

Not Presented at 2013 AMR

Photo Credit: Jana Jeffers, NREL
10

Technical Accomplishments

Baseline 18 AWG 22 AWG 26 AWG

Radius (wire and insulation), 

mm

N/A 0.547 0.351 0.226

Total wetted surface area, mm2 126.7 148.2 143.3 139.2

• ATF impingement baseline target is flat polished copper with 600-grit sandpaper

• Additional targets mimic wire bundles with insulation (18, 22, and 26 AWG)

AWG = American Wire Gauge

18 AWG 22 AWG 26 AWG

FY13 Related Accomplishments 

Not Presented at 2013 AMR

Credit: Gilbert Moreno, NREL
11

Technical Accomplishments

Heat transfer coefficients of all target 

surfaces at 50°C inlet temperature

• At lower impingement velocities, 

all samples achieve similar heat 

transfer

18 AWG surface target

FY13 Related Accomplishments 

Not Presented at 2013 AMR

Note: Heat transfer coefficient calculated from the base projected area (not wetted area)

Top View

Side View

Photo Credit: Gilbert Moreno, NREL

Photo Credit: Gilbert Moreno, NREL

50°C Inlet Temperature

0

2000

4000

6000

8000

10000

12000

0 2 4 6 8 10 12

Heat Transfer Coefficient [W/m

2K]

Velocity [m/s]

18 AWG

22 AWG

26 AWG

Baseline
12

Technical Accomplishments

• Fluid splatter observed at higher velocities for 70°C and 90°C inlet liquid temperatures

• Fluid splatter more prevalent at higher temperatures

• Splatter at 90°C occurred at lower velocity than at 70°C

• As temperature increases, it is expected that the fluid splatter will occur at lower velocities

ATF flowing over surface

ATF deflecting off surface

Note: ATF viscosity decreases as temperature increases

FY13 Related Accomplishments 

Not Presented at 2013 AMR

Photo Credit: Jana Jeffers, NREL

Photo Credit: Jana Jeffers, NREL

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2 4 6 8 10 12

Heat Transfer Coefficient [W/m

2K]

Velocity [m/s]

50°C

70°C

90°C

18 AWG sample data for all inlet temperatures
13

Technical Accomplishments

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

0

500

1000

1500

2000

2500

0 2 4 6 8 10

Measured Lamination Stack 

Thermal Resistance (mm2-K/W]

Number of Contacts

M19 29 Gauge, 138 kPa Data

Data with Systematic Error

Weighted Curve Fit

• Lamination-to-lamination thermal contact resistance calculated from 

slope of weighted curve fit

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader

Copper Metering Block

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

Error bars represent 95% confidence level
14

Technical Accomplishments

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

• The lamination-to-lamination thermal contact resistance appears to be affected by 

the surface roughness

• The effective through-stack thermal conductivity is calculated from:

o Lamination-to-lamination thermal contact resistance

o Bulk lamination material thermal resistance

o Number of laminations 

0

50

100

150

200

250

300

350

138 276 414 552

Lamination-to-Lamination 

Thermal Contact Resistance 

[mm2-K/W]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level
15

Technical Accomplishments

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

0 10 20 30 40 50

Thermal Conductivity [W/m-K]

Number of Laminations (NL)

M19 29 Gauge, 138 kPa Example

Effective Through-Stack Thermal Conductivity

Asymptote (NL → ∞)

• The effective through-stack thermal conductivity approaches the 

asymptote within 30–50 laminations
16

Technical Accomplishments

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 200 400 600

Asymptotic Stack Thermal Conductivity 

[W/m-K]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level
17

Technical Accomplishments

• Measured in-plane thermal conductivity of 

lamination stacks provided by ORNL

1. Based on measured thermal conductivity of similar material

2. Calculated assuming 99% stacking factor

3. Average of measured orthotropic property in setup shown in figure

21.9 21.7 21.8

0.0

5.0

10.0

15.0

20.0

25.0

Bulk

Material

Calculated

Value

Measured

In-Plane Thermal Conductivity 

[W/m-K]

M19 29 Gauge

1 2

3

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader

Copper Metering Block

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

• Confirmed in-plane thermal conductivity is 

close to bulk material thermal conductivity

Photo Credit: Justin Cousineau, NREL
18

Technical Accomplishments

• Measured cross-slot 

thermal conductivity of 

wire bundles prepared and 

provided by ORNL

Note: Wire fill factor includes copper and insulation

0.48

1.06

0.66

0.54

0.98

0.60

0

0.2

0.4

0.6

0.8

1

1.2

Test Results Model:

Perfect Fill

Factor

Model: 50%

Voiding

Cross-Slot Thermal Conductivity 

(W/m-K)

Wire Fill Factor: 0.67 Wire Fill Factor: 0.63

• The agreement between model and 

experimental results depend on assumptions 

for fill factor and voiding

• Modeling approach appears to match but 

additional testing is needed

Photo Credits: Justin Cousineau, NREL
19

Response to Previous Year Reviewers’ Comments

This project is a new project for FY14 but reviewer input from 

previous motor thermal research efforts have guided the focus 

of the current project.

• Reviewers mentioned oil impingement cooling is an effective method for 

cooling and a better understanding of this cooling technique is of high 

interest for automotive applications.

– A focus of this new project includes experimental methods to measure and 

understand oil impingement cooling.

• It was mentioned of past work that “passive thermal design elements are a 

unique enabler.”

– The second focus area of this project is to work with partners to experimentally 

characterize passive thermal design elements, which include thermal properties of 

interfaces and materials for motor applications.
20

Collaboration and Coordination with Other Institutions

• Industry

o Motor industry suppliers, end users, and researchers

– Input on research and test plans

– Sharing of experimental data, modeling results, and analysis methods

– Companies providing research input, requesting data, or supplying data 

include: Ford, Chrysler, Tesla, UQM Technologies, Remy, Magna

• Other Government Laboratories

o ORNL

– Support from benchmarking activities

– Collaboration on motor designs to reduce or eliminate rare-earth 

materials

– Collaboration on materials with improved thermal properties

 Potting materials for end windings for improved heat transfer

 Slot winding materials
21

Remaining Challenges and Barriers

Cooling Technology 

Development

• Thermal tests of interfaces and materials for motor cooling

o Slot-windings

o End-windings

• In-situ thermal resistance measurements

• Correlations of ATF impingement cooling on motor windings

• CFD simulation of ATF impingement cooling

• Variation in local heat transfer coefficients of ATF impingement

• Effective convective heat transfer coefficients for representative 

end-windings

Passive Thermal Stack 

and Reliability

Credit: Gilbert Moreno, NREL

Credit: Gilbert Moreno, NREL

Credit: Sreekant Narumanchi, NREL Credit: Doug Devoto, NREL
22

Proposed Future Work

Ongoing

• Continue ongoing collaboration with ORNL material 

developments and motor research

• Measurements of thermal interfaces and effective orthotropic 

thermal properties of materials (windings and potting 

materials)

FY14 

• Develop convection coefficient heat transfer correlations using 

ATF fluid property information provided by Ford

• Develop methods for simulating impinging ATF fluid jets 

validated against experimental data

• Utilize infrared imaging and thermochromic liquid crystal (TLC) 

technology to measure local heat transfer coefficients of 

impinging ATF fluid jets

• Develop test methods and begin heat transfer measurements 

on representative end-winding features

FY15

• Measure effective heat transfer coefficients for representative 

end-windings

• Measure in-situ thermal resistances

Sample TLC Color 

Representations for Surface 

Temperature

ATF Cooling of Representative 

Motor End-Winding

Photo Credit: Jana Jeffers, NREL

Photo Credit: Kevin Bennion, NREL
23

Summary

Relevance

• Supports transition to more electric-drive vehicles with higher continuous power requirements

• Enables improved performance of non-rare earth motors and supports lower cost through reduction 

of rare earth materials used to meet temperature requirements (dysprosium)

Approach/Strategy

• Engage in collaborations with motor design experts within industry

• Collaborate with ORNL to provide motor thermal analysis support on related motor research at ORNL

• Perform in-house thermal characterization of materials, interface thermal properties, and cooling 

techniques

Technical Accomplishments

• Measured ATF heat transfer convection coefficients on target surfaces

• Received materials from ORNL and measured orthotropic thermal conductivity

• Completed expanded lamination thermal tests for publication

• Received ATF fluid property data from Ford Motor Company to support future work to develop 

correlations and CFD models

Collaborations

• Motor industry representatives: manufacturers, researchers, and end users (light-duty and 

medium/heavy-duty applications)

• Oak Ridge National Laboratory
For more information, contact:

Principal Investigator

Kevin Bennion

Kevin.Bennion@nrel.gov

Phone: (303)-275-4447 

APEEM Task Leader:

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303)-275-4062

Acknowledgments:

Susan Rogers and Steven Boyd, 

U.S. Department of Energy

Team Members:

Justin Cousineau (NREL)

Jana Jeffers (NREL)

Charlie King (NREL)

Gilbert Moreno (NREL)

Tim Burress (ORNL)

Andy Wereszczak (ORNL)
Technical Back-Up Slides
26

Lamination Stacking Factor and Pressure

Error bars represent ±U95 uncertainty

• Graph shows relationship between stacking factor and the applied 

pressure on the stack of laminations

0.96

0.97

0.98

0.99

1.00

1.01

0 100 200 300 400 500 600

Stacking Factor

Pressure [kPa]

15 Laminations M19-29 Gauge